porphyria cutanea tarda (PCT), the most common form of porphyria, characterized by cutaneous photosensitivity that causes scarring bullae, hyperpigmentation, facial hypertrichosis, and sometimes sclerodermatous thickenings and alopecia. It is frequently associated with alcohol abuse, liver disease, or hepatic siderosis. Urinary levels of uroporphyrin and coproporphyrin are increased, and activity of a specific enzyme involved in heme biosynthesis is decreased. The cause is debated, but two types are generally recognized: an autosomal-dominant (or familial) form in which activity of the affected enzyme is reduced to half normal in liver, erythrocytes, and fibroblasts, and a sporadic (but probably also familial) form in which the reduction is confined to the liver. Both types are believed to be heterozygous, with clinical expression occurring in adulthood, precipitated by disease or environmental factors. A more severe homozygous form begins in childhood and is called hepatoerythropoietic porphyria. See also porphyria, hepatoerythropoietic porphyria.
